1. Field of the Invention
The present invention relates generally to a hollow blow-molded container, and more particularly to a uniquely shaped blow-molded container able to accommodate a hot-fill and sealing process without any apparent adverse effects on the container's tactile feel or visual appearance.
2. Description of Related Art
Lightweight containers made of thermoplastic materials such as polyester, polyamide, and polyolefin resin and other thermoplastic polymers are well known in the container industry. Polyester containers produced by the conventional molding process, however, exhibit extremely high thermal distortion which makes them unsuitable for the packaging of products which require filling at elevated temperatures.
In most packaging facilities the techniques and apparatus presently employed require that a filled container be capped and sealed immediately after the filling operation (while the contents are still hot). The contents in the sealed container and the warmed head space shrink as they cool, resulting in a partial vacuum being created inside the container. Resulting pressure differentials create a net pressure force on the outside of the container walls which can cause the container to buckle or collapse. This uncontrolled buckling is aesthetically unattractive and renders the containers commercially unacceptable. While containers can be stiffened, e.g., with integrally molded ribs and the like or by increasing the wall thickness, these techniques are not always practical to produce a container which can resist the vacuum-induced buckling forces generated in hot-fill applications. Small containers, which have less surface area for structural reinforcement, present a particular problem. It can be difficult to design a small container that is aesthetically pleasing and structurally sound.
Deformation upon the hot-filling and sealing in a container results from two distinct phenomena. The first is a thermal phenomenon. When the hot contents contact the polyester container, the container walls shrink, usually unevenly, causing distortion of the container. Thermal stabilization alone, however, is not sufficient to render a plastic bottle suitable for most commercial hot-fill applications, in which capping is effected immediately after filling to facilitate high speed processing.
The second deforming phenomenon present in hot-fill applications occurs when a thermoplastic container is filled with a hot liquid (such as a liquid sterilized at a high temperature) and sealed. Subsequent thermal contraction of the liquid upon cooling results in partial evacuation of the container which tends to deform the container walls. Backflow into a filling mechanism and the use of vacuum filling equipment during filling operations can similarly create a partial vacuum inside the container resulting in its deformation. Such deformation typically concentrates at the mechanically weaker portions of the container, resulting in an irregular and commercially unacceptable appearance. Further, if the deformation occurs in an area where the label is attached to the container, the appearance of the label may be adversely affected as a result of container deformation.
In order to avoid collapse from these internal vacuum forces it is necessary to either produce a container which is sufficiently rigid to withstand forces of this magnitude or to provide for a reduction in the container volume to offset the volume change during cooling. Practical limitations in the manufacture of plastic containers, however, prevent the production of commercially acceptable containers of sufficient rigidity to withstand these pressure forces.
By increasing the wall thickness of the container it is possible to some extent to strengthen the container walls and thus decrease the effects of vacuum deformation. However, increasing the wall thickness results in a substantial increase in the amount of raw materials required to produce the container and a substantial decrease in production speed. The resultant increased costs are not acceptable to the container industry. Additionally, increase in wall thickness results in decrease in bottle fill capacity.
A prior attempt to reduce the effects of vacuum deformation is disclosed in U.S. Pat. No. 3,708,082 to Platte. Platte discloses a plastic container with four flat wall-panels comprising the body portion of the container. A rib circumscribes the entire container in a region below the handle and serves to rigidify the side wall-portions in a circumferential direction. The rib also acts as a hinge to allow limited inward collapsing of the container along selected regions.
Another prior approach to reduction of the effects of vacuum deformation is to provide a container with a plurality of recessed collapse panels, separated by lands, which allow uniform controlled inward deformation so that vacuum effects are accommodated in a uniform manner without adverse effects on the appearance of the container. A container having such vacuum flex panels is disclosed in International Publication No. WO 00/50309 (Melrose), which is incorporated herein by reference. The container has a controlled deflection vacuum flex panel that inverts and flexes under pressure to avoid deformation and permanent buckling of the container. It includes an initiator portion, which has a lesser projection than the remainder of the flex panel and initiates deflection of the flex panel.
U.S. Pat. No. 4,877,141 to Hayashi et al. discloses a pressure resistant bottle shaped container having panels with stress absorbing strips. The panels prevent permanent deformation that result from pressure changes when the container is filled with high temperature liquids. U.S. Pat. Nos. 5,141,120 and 5,141,121 to Brown et al. both of which are hereby incorporated by reference, disclose a hot fill container having opposing pinch grip indentations in the sidewall. The indentations collapse inwardly toward each other to accommodate internal forces that result from filling the container with high temperature liquid. Another example of containers having such vacuum flex panels is disclosed in U.S. Pat. No. 5,392,937 and Des. 344,457 to Prevot et al., both of which are assigned to the assignee of the present invention and are hereby incorporated by reference. In these containers, a grip structure moves with the vacuum flex panel in response to the vacuum created inside the container in response to hot-filling, capping, and cooling of container contents.
U.S. Pat. No. 4,732,455 to Cochran discloses a lightweight thermoplastic container having four flat sidewalls connected by curved corner portions and a bottom portion connected to the flat sidewalls by curved base portions. The container also has longitudinally extending ribbing structures in opposing corner portions to withstand hydrostatic forces without buckling or dimpling.
Agrawal et al., U.S. Pat. No. 4,497,855 discloses a container with a plurality of recessed collapse panels, separated by land areas, which allows uniformly inward deformation under vacuum force. The vacuum effects are controlled without adversely affecting the appearance of the container. The panels are drawn inwardly to vent the internal vacuum and so prevent excess force being applied to the container structure, which would otherwise deform the inflexible post or land area structures. The amount of “flex” available in each panel is limited, however, and as the limit is approached there is an increased amount of force that is transferred to the side walls.
U.S. Pat. No. 4,298,045 to Weiler et al. shows another prior art approach in which a thermoplastic container has rigidifying grooves and embossments provided in the side walls of the container. Rather than controlling collapse, these rigidifying features substantially eliminate collapse, and are thus useful only with relatively low levels of evacuation.
Prior art approaches, including the use of flex or collapse panels to overcome thermal deformation are not without problems. While collapse panels accommodate a great degree of controlled deformation, as the vacuum inside the containers increases, more and more collapse is required from the collapse panels without permitting collapse of the intervening lands. By increasing the length of the corner step of the collapse panels the rigidity of the lands may be increased. However, the resultant deeper collapse panel occupies a larger internal volume of the container, and the overflow capacity of the container is significantly decreased. In order to compensate for this decrease in overflow capacity, the container diameter must be increased. Any increase in container diameter, however, decreases container rigidity. Thus, any container rigidity gained by increase in the size of the collapse panel is offset by the need to regain the lost overflow capacity. The present invention eliminates the aforementioned disadvantages.